1. Field of the Invention
The present invention relates to an intake port for smoothly performing combustion over the entire operating range in a four-stroke cycle internal combustion engine in which one of a pair of intake valves is in a resting condition (incidentally, in the present specification, even when the lift amount of the intake valve is minute and the intake of a slight amount of air or air-fuel mixture is performed, the intake valve is referred to as xe2x80x9cbeing in a resting conditionxe2x80x9d), for suppressing the emission of air pollutants, which are contained in exhaust gases, therefrom as much as possible, and for obtaining high output and efficiency.
2. Description of the Related Art
As the technique for enhancing thermal efficiency to reduce combustion emissions in a four-cycle internal combustion engine, there has been developed a method of causing a horizontal swirl whirling about the central line or axis of a cylinder in intake air or air-fuel mixture flowing to a combustion chamber in the cylinder in the range of a low intake air operation, such as a low load operation or a low speed operation, thereby improving the combustion condition.
Hitherto, as means for generating a swirl, there has been developed a device for enhancing the directivity of an intake airflow in the combustion chamber in the low intake air operating range during the resting condition of the intake valve by extremely largely bending an intake-valve normally-operating side branch port, of which intake valves continuously open and close, between a pair of branch intake ports each having a couple of intake valves.
Further, as other means for generating a swirl, there have been developed devices for letting intake air, which flows through the intake-valve resting side branch port, flow into the intake-valve normally-operating side branch port through the communication hole from a side thereof in a low intake air operating range during the resting condition of the intake valve by providing a communication hole in a partition wall between a pair of an intake-valve normally-operating side branch port and an intake-valve resting side branch port, thereby enhancing a swirl (see JP-B-59-33852U and JP-A-6-272561 official gazettes).
Although a swirl is enhanced by increasing the curvature of the intake-valve normally-operating side branch port or decreasing the transversal sectional area thereof so as to improve the combustion condition and reduce fuel consumption, the intake resistance increases in a high intake air operating range in which both the intake valves are opened and closed. Thus, the conventional devices have drawbacks in that the intake flow coefficient is degraded, that the maximum output of the engine in the high intake air operating condition decreases, and that the fuel consumption thereof is deteriorated.
The object of the present invention is to provide an intake port for smoothly performing combustion over the entire operating range in a four-stroke cycle internal combustion engine while eliminating such drawbacks.
According to the present invention, there is provided a first intake port structure in a four-stroke cycle internal combustion engine. In this engine, a common intake port is branched into a pair of branch intake ports, which are separated through a partition wall, at the downstream end thereof. Further, a pair of intake valves adapted to independently and respectively open and close the pair of branch intake ports are provided therein. Moreover, one of the pair of intake valves opens and closes in a low intake air operating range, while the other intake valve is in a resting condition. Furthermore, both the pair of intake valves open and close in a high intake air operating range. The intake port structure comprises a cutout formed in the laterally central portion of the partition wall in such a way as to extend from an edge of the upstream end of the partition wall toward the downstream end thereof and to separate the pair of branch intake ports from each other.
In the case of this intake port structure, the cutout is formed extending from the edge of the upstream end of a partition wall is formed in a laterally central portion of the partition wall that separates the pair of branch intake ports, as described above. Therefore, in a high intake air operating range in which both the intake valves open and close, no intake airflow is largely disturbed by this cutout. Moreover, the sectional area of each of the branch intake ports is not reduced. This enables increase in the maximum output and improvement of fuel efficiency. Furthermore, in a low intake air operating range in which one of the intake valves is in a resting condition, intake air having flown into the intake-valve resting side branch intake port flows into a normally-operating side branch intake port through the cutout without flowing backward to the upstream common intake port. Thus, this intake air joins intake air contained in the normally-operating side branch intake port. This enhances a swirl of intake air flowing into a combustion chamber from the normally-operating side branch intake port. Consequently, the concentration of air-fuel mixture in the intake air is uniformed in the low intake air operating condition, such as the low load operating condition or the low speed operating condition. Moreover, the generation of NOx gas and unburnt gas is suppressed. Furthermore, lean combustibility is enhanced. Consequently, the improvement of fuel efficiency is enabled.
Further, apart of the partition wall is removed by forming the cutout, so that the area of the surface of the partition wall decreases. Thus, the deposition of fuel on the wall surface increases. Consequently, the startability of the internal combustion engine is enhanced. Moreover, the properties of exhaust gas are improved.
Furthermore, according to the intake port structure of the present invention, the downstream end surface of the cutout formed in the partition wall is inclined in a direction in which the downstream end surface thereof approaches an opening of one of the intake valves that continuously open and close. Therefore, even in any intake air operating condition, especially, in a low intake air operating condition, intake air having flown through the cutout portion to the downstream end thereof smoothly flows into the normally-operating side branch port. Thus, a swirl is enhanced still more without reducing the intake flow coefficient. Consequently, the effects of the first intake port structure are significantly enhanced.
Further, according to the intake port structure of the present invention, the groove portion extending from the upstream side of the intake-valve resting side branch intake port to the downstream side of the intake-valve normally-operating side branch port is formed in a curved inner surface portion of the pair of branch intake ports at the intake valve-opening-side of the cutout provided in the partition wall. Therefore, in a low intake air operating condition, intake air flowing through the intake-valve resting side branch intake port into the groove in the cutout can be led to the vicinity of an intake opening of the intake-valve normally-operating side branch intake port. Thus, a swirl can be enhanced still more. The generation of NOx gas can be suppressed still more, and the lean combustibility can be improved still more.
Furthermore, according to the intake port structure of the present invention, the bottom surface of the groove portion smoothly connects with the bottom surface of the intake-valve resting side branch intake port and the downstream bottom surface of the intake-valve normally-operating side branch intake port. Thus, in a low intake air operating condition, intake air flowing from the intake-valve resting side branch intake port can be made to smoothly flow through the groove of the cutout to the intake-valve normally-operating side branch intake port. Consequently, the effects of the third intake port structure can be enhanced still more.
Additionally, according to the intake port structure of the present invention, the transverse sectional area of the intake-valve normally-operating side branch intake port is set in such a manner as to be smaller than the transverse sectional area of the intake-valve resting side branch intake port. Therefore, in a low intake air operating condition, a swirl can be enhanced still more by increasing the flow velocity of intake air flowing through the intake-valve normally-operating side branch intake port. Thus, the generation of NOx gas and unburnt gas is suppressed still more. Consequently, the fuel efficiency is improved still more.
Further, according to the intake port structure of the present invention, each of the side surface of the partition wall and the side surface of the intake-valve normally-operating side branch intake port, which faces the side surface of the partition wall, is set in such a manner as to be narrower than the width between the other side surface of the partition wall and the side surface of the intake-valve resting side branch intake port, which surface faces the latter side surface of the partition wall. Thus, the effects of the fifth embodiment can be enhanced still more.